Kim Feinknopf-Dorrian offered considerable, and excellent, advice. I'll
add only a few additional items.
First, examine the motive for cleaning the stones. There are biologicals
that damage stone and should be removed. Yet, much cleaning is simply
because local groups want to "do something." Much of this cleaning is
far too aggressive. There are several good studies (admittedly on
architectural stone) which suggest that cleaning may actually increase
the rate of future soiling. Moreover, we try to emphasize that cleaning
-- if undertaken -- soon becomes a routine maintenance activity. So you
must ask yourself (1) is this really in the best interests of the stone
and (2) is this where we want to place our preservation priorities. We
may want to question whether some stones are soiled or whether they have
developed a rich patina.
Second, recognize that there are a variety of cleaning products (Kim
mentioned photo-flo and Triton X 100; there is also vulpex, often used
on stone). Each has certain characteristics and tends to be appropriate
under differing circumstances. There is no one "right" material
(although water alone comes the closest) for all stones any more than
there is one "right" finish for furniture or one "right" technique for
cleaning fabrics. Until you have the ability to recognize when to use
different materials, remember the warning in conservation treatments,
"do no harm."
Third, recognize that many traditional cleaners do considerable harm
(and even appropriate cleaners can harm unless they are thoroughly
rinsed not only "off" the stone, but also "out of" the stone). For
example, household bleach (sodium hypochlorite), which regrettably many
people use to "restore" the brilliant "whiteness" of marble can be very
damaging. It can form soluble salts, which reappear as white
efflorescence and it can cause yellowing of the stone. Calcium
hypolchlorite has similar side effects (and the residual salts are also
acidic). Even use of household ammonia may lead to discoloration of the
stone.
Fourth, there are some biocides out there that have been and are being
used and/or tested in stone conservation. For example, there is a new
product, D/2 Architectural Biocide. It seems to be effective. It seems
to rinse off and out easily. It seems to have a pH appropriate for many
stones. But I would still caution that it should be used sparingly and
cautiously. For any product we need considerable evidence of safe,
long-term use before it becomes more widely recommended -- this is why,
in general, conservation treatments tend to be conservative, relying on
techniques and materials which have a long history of success.
There are several upcoming workshops on cemetery preservation issues --
one on October 29 in Virginia sponsored by the Virginia Association of
Museums (contact Susan Brown at VAM for more information, 804/649-8261).
And there is another at Restoration 99 in Charleston, SC (contact EGI
Exhibitions, 800/982-6247). These are the sorts of issues that both
workshops will address.
Best,
Michael Trinkley, Ph.D.
Director
Chicora Foundation, Inc.
PO Box 8664
Columbia, SC 29202
803/787-6910
Email: [log in to unmask]
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